Security threads embedded in the substrate are known to the skilled person as an efficient means for the protection of security documents and banknotes against imitation. Reference is made to U.S. Pat. No. 0,964,014; U.S. Pat. No. 4,652,015; U.S. Pat. No. 5,068,008; U.S. Pat. No. 5,324,079; WO 90/08367; WO 92/11142; WO 96/04143; WO 96/39685; WO 98/19866; EP-A 0 021 350; EP-A 0 185 396; EP-A 0 303 725; EP-A 0 319 157; EP-A 0 518 140; EP-A 0 608 078; EP-A 0 635 431; and EP-A 1 498 545 as well as the references cited therein.
A security thread is a metal- or plastic-filament, which is incorporated during the manufacturing process into the substrate serving for printing security documents or banknotes. The security thread may hereby be completely embedded within the substrate sheet, or it may be partly embedded and partly exposed at the surface of the substrate (“window-thread”), or it may even be affixed to the surface of the substrate or bridge two separate parts of the substrate sheet; such threads are also called stripes.
A security thread or stripe may, and does in general, carry particular security elements, serving for the public and/or machine-authentication of the security document, in particular for banknotes. Suitable security elements for such purpose are e.g. a metallization, a luminescent compound (incorporated into, or printed onto the thread or stripe), a micro-text, a magnetic feature, etc.
Due to the technical constraints of industrial manufacturing, the security thread or stripe must be incorporated from a reel into an endless sheet of substrate material, such as currency paper, being several hundred meters in length. Such thread or stripe is generally produced through a corresponding slicing of a web of a particularly treated (i.e. metallized, imprinted, laminated, etc.) plastic foil (such as a mono- or bi-oriented polypropylene (PP), a polyvinylchloride (PVC), or a polyethylene-terephthalate (PET) foil), to yield the required reels of security thread or stripe.
In a common embodiment, said plastic foil is metallized and/or imprinted on a single side. The metallization may furthermore be present in the form of indicia in positive or negative writing. In a more sophisticated embodiment, said plastic foil is a laminated structure, consisting of two foils which are laminated together, enclosing security elements such as a printed feature and/or a metallization, between two plastic foils.
Optically variable magnetic pigment (OVMP®) and optically variable magnetic inks (OVMI®) comprising OVMP® are known to the skilled person from e.g. U.S. Pat. No. 4,838,648; WO 02/073250; EP 686 675; WO 03/00801; U.S. Pat. No. 6,838,166; and WO 2007/131833. Such inks can be applied or printed in the form of plain coatings (i.e. covering a whole surface) or in the form of structured coatings (i.e. indicia).
The optically variable pigment particles in an optically variable magnetic coating can be oriented after printing, while the coating is still “wet”, i.e. unhardened, through the application of an appropriate unstructured or structured magnetic field, and then fixed in their respective positions and orientations through a hardening of the coating composition on the substrate. Materials and technology for the orienting of magnetic or magnetisable particles in a coating composition, and corresponding combined printing/magnetic orienting processes have been disclosed in U.S. Pat. No. 2,418,479; U.S. Pat. No. 2,570,856; U.S. Pat. No. 3,791,864; DE 2006848-A; U.S. Pat. No. 3,676,273; U.S. Pat. No. 5,364,689; U.S. Pat. No. 6,103,361; US 2004/0051297; US 2004/0009309; EP-A-710508, WO 02/090002; WO 03/000801; WO 2005/002866, and US 2002/0160194, as well as WO 2008/046702 of the same applicant.
Items comprising magnetically oriented particles in a hardened coating on a transparent or opaque substrate are known in the art, e.g. from WO 2008/009569. However, a particular technical problem arises in the case of security threads or stripes, in that they must i) display a seamless repetitive pattern in their elongated direction, and ii) the repetition length (period) of said pattern must be smaller than the width, preferably smaller than the half width of the banknote or the security document into which they are incorporated.
These requirements assure that i) there is no need for cutting away parts of the security document substrate, nor for aligning the banknote or security document printing with the information present on the thread or stripe, because the latter is continuous (without jumps) along the security thread or stripe, and that ii) at least one whole period of the information present on the thread or stripe is actually present on/in each banknote or security document, allowing for an unambiguous authentication of the latter.
These requirements have not been previously resolved for magnetically oriented particles in a hardened coating on a substrate.